Heating appliances with an improved efficiency (IE) and high efficiency (HE) are in large scale use in the Netherlands and elsewhere for heating dwellings and other accommodation areas. For many dwellings it is important that such heating appliances take the most compact possible form, while it must be avoided that the appliances are so complicated that regular, costly servicing and maintenance has to take place.
These known heating appliances have a number of drawbacks, a number of significant ones being: the size of the appliances, whereby they are difficult to build into a cupboard or the like; the emission of CO and NO.sub.x as well as the associated efficiency loss, for instance because considerable temperature differences are present inside the apparatus.